1. Field of the Invention
This invention is concerned with a unique operation of a roller grinding mill to process granular fuel material to a predetermined particle size with a minimum of air and without a classifier unit.
2. Description of the Prior Art
As far as is known, roller grinding mills, almost from the very first, were constructed so that the grinding rollers and the cooperating plows were driven in a circular path in a circular grinding chamber so that both the rollers and plows moved in the direction of the air admitted to the grinding chamber or at least perpendicular (across) to the flow of the air entering the grinding chamber. It was never recognized that this long-standing arrangement of the plows and air inlet provisions caused the plows to centrifuge the material into the air and away from between the grinding rolls and the bull ring means to the detriment of particle size control. The answer to that problem almost universally has been that roller mills had to be provided with some sort of classifier device outside of the mill to obtain the desired sort of material particle size, and so the desired control over the particle size of the ground material that was allowed to pass into the outlet duct system was obtained. That arrangement was satisfactory as long as the output delivery to a burner was combined with its own secondary combustion supporting air.
When the handling of fuel material went to the stage that direct supply to a burner was demanded, a wholly different system of treating the fuel material was needed. When fuel material was processed in the mill and delivered directly to the burner, problems arose in the need to supply an excess of air into the mill to control the particle size and that was found to be getting too much air at the burner, which was an inefficient way to fire a burner, and the products of combustion became troublesome pollutants in the atmosphere.
Typical examples of older conventional types of roller grinding and pulverizing mills are seen in U.S. Pat. Nos. 1,769,352 of July 1, 1930 and 1,936,593 of Nov. 28, 1930. In these mills the rollers and plows travel in a direction (usually clockwise) with the flow of the air delivered into the grinding chamber so that the material lifted up to be crushed occurs in front of the rollers. However, that lifting of the material is offset and greatly neutralized by the pitch of the plows catching the air flow on the back side with the result that the air is diverted t the outside. Since the air velocity is higher than the plow velocity, the slower plows create sudden formation of disturbances in the airflow similar to the cavitation effect in fluid flow which generates turbulence that causes movement of the material to the outside surface of the grinding chamber under centrifugal forces acting on the material. It is believed this results in pulverizing the material to a fine particle size as it remains longer in the grinding chamber.
The conventional roller mills are formed to require more air in order to lift the particles, and the particle sizes are not uniform so an extra piece of equipment in the form of a classifier is needed in the mill output to return oversize particles to the mill for further reduction.